How to get a list of specific fields values from objects stored in a list?

Question

Say I have an list of objects with two fields field1 and field2, both of String type.

How do I get a list of all field1 values wit

Answer 1

Add the values of records in different lists and by using the one or two iterator simply print the seperate values.like follows:

rs=st.executeQuery("select * from stu");
List data=new ArrayList();
List data1=new ArrayList();
while(rs.next())
{
   data.add(rs.getString(1));
   data1.add(rs.getString(2));
}
Iterator it=data.iterator();
Iterator it1=data1.iterator();
while(it1.hasNext())
{
   System.out.println(" "+it.next()+"  "+it1.next());
}

Answer 2

Ancient question, but I came upon it while looking to see if I could improve on a similar solution.

You can implement the List<String> interface without creating a fleshed-out ArrayList<String>, and thus not iterating over the parent object.

final List<Entity> entities = getEntities()
final List<String> field1 = new AbstractList() {
    public String get(int index) {
        return entities.get(index).getField1();
    }
    public int size() {
        return entities.size();
    }
}

That gives you a List without iterating over the parent object.

Random access to the derived List<String> will be as expensive as random access to the underlying List<Entity>; if you're using an implementation of List<Entity> that does not provide quick random access, you might have to jump through a couple of hoops (i.e. implementing more methods of List<String>. But this should work for 99% of the cases where you need a light-weight adapter.

Answer 3

Let the object be of the following class.

public class Bike {

    String bikeModel;
    Int price;
}

And now there is list of bikes called bikeList of type List

So now we want a list of bikemodels of all the bikes in the above list.

bikeList.map{ Bike b -> b.bikeModel }.toCollection(arrayListOf())

returns an array list of first field of all the bike objects in the bikeList

Answer 4

try this:

List<Entity> entities = getEntities();
List<Integer> listIntegerEntities = Lambda.extract(entities, Lambda.on(Entity.class).getFielf1());

the LambdaJ allows to access collections without explicit loops, so instead of have more lines of code to iterate the list yourself, you let LambdaJ do it.

Answer 5

Neither java as a language nor JDK libraries do not do what you want yet. You can either use LambdaJ or wait for Java 8 that is expected to include lambda expressions.

Answer 6

Fortunately, you can do this using Java 8 - Streams

Assume you have an entity named YourEntity

public class YourEntity {

    private String field1;
    private String field2;

    public YourEntity(String field1, String field2) {
        this.field1 = field1;
        this.field2 = field2;
    }

    public void setField1(String field1) {
        this.field1 = field1;
    }

    public void setField2(String field2) {
        this.field2 = field2;
    }

    public String getField1() {
        return field1;
    }

    public String getField2() {
        return field2;
    }
}

Declare you list of YourEntity using:

List<YourEntity> entities = Arrays.asList(new YourEntity("text1", "text2"), new YourEntity("text3", "text4"));

You can extract the list of field1 in one shot in this way:

import java.util.stream.Collectors;

List<String> field1List = entities.stream().map(YourEntity::getField1).collect(Collectors.toList());

Or in this way

import java.util.stream.Collectors;

List<String> field1List = entities.stream().map(urEntity -> urEntity.getField1()).collect(Collectors.toList());

You can print all the items also using java 8 :)

field1List.forEach(System.out::println);

Output

text1
text3